Wire pit

ABSTRACT

The present invention relates to a wire pit. A characteristic feature of the wire pit ( 50 ) according to the invention is that its wall/walls ( 52′, 52″, 54′, 62 ) converge downwards so that the average flow direction of the liquid along the most part of the wire pit&#39;s ( 50 ) height deviates from vertical.

This application is the U.S. national phase of international applicationPCT/F100/00695 filed Aug. 17, 2000, which designated the U.S.

The present invention relates to a wire pit. Especially preferably theinvention relates to a new kind of wire pit construction having awall/walls converging downwards so that the average flow direction ofthe liquid at the most part of the wire pit's height deviates fromvertical.

Almost all prior art paper machine approach systems feeding paper pulpto the paper machine, which are well described e.g. in U.S. Pat. No.4,219,340, comprise the following components: A white water tank, acentrifugal cleaning plant with feed pumps and pumps between variousstages, a gas-separation tank with vacuum providing means, a head boxfeed pump, a head box screen, a paper machine head box and white watertrays. Said components are placed in connection with the paper machineand arranged to operate as is follows. The fiber material used for papermaking and the fillers which are diluted with so-called white waterobtained from the wire section of the paper machine are dosed by meansof a basis weight regulation valve from the machine chest into the whitewater tank usually located at the bottom level of the mill. By means ofa feed pump also located at the bottom level of the mill, the fibersuspension is pumped from the white water tank to the first cleaningstage of a centrifugal cleaning plant usually located at the machinelevel, i.e. the location level of the paper machine, or, as in saidpatent, above it. The centrifugal cleaning plant most typicallycomprises several (most commonly 4-6) stages each typically having afeed pump of its own. By means of pressure created by said feed pump,the fiber suspension accepted in the first cleaning stage of thecentrifugal cleaning plant is further conveyed to a gas-separation tanktypically located at a level above the machine level. In thegas-separation tank, the fiber suspension is subjected to the effect ofvacuum created by means of vacuum providing apparatus, most usuallyliquid rings pumps, whereby both part of the gas dissolved in thesuspension and the gas existing in the suspension in small bubbles risesabove the surface of the liquid in the tank and is discharged from thetank via the vacuum providing apparatus. From the gas-separation tankthe fiber suspension, wherefrom gas has been removed as thoroughly aspossible, flows to a head box feed pump located at the bottom level ofthe mill, which pump further pumps the fiber suspension to a head boxscreen (not shown in said U.S. patent) also located at the bottom levelof the mill, whereafter the fiber suspension flows to the machine levelinto the head box of the paper machine.

One problem in the prior art paper machine approach system is its hugevolume mostly due to the volume of the gas-separation tank and thecentrifugal cleaning plant as well as the long and large-sized piping.Volume in itself is not a major problem, except for space requirementand being a relatively big investment, but long delays caused by greatvolumes substantially restrain the change of grade and result in greatamounts of broke in connection with the changes of grade. In connectionwith the grade change, broke is formed of all the pulp being used toproduce the final product before the relative amounts of all componentsof the fiber suspension have been equalized throughout the approachsystem to correspond to the content of the desired final product.

Said problem has already been dealt with in FI patent 89728, accordingto which different types of white waters are collected from the wiresection of the paper machine and guided directly to the shortcirculation of the paper machine without employing any actual whitewater tank. In said publication, under each white water tray there is apump for delivering the white water to a suitable location. Thepublication describes the white water channels to be very flat, i.e. ofsmall volume, so that the delays remain as short as possible. In thesolution according to said publication, arranged at the side of the wiresection there is a small pumping container and means providing pumpingoperation, from which the white water is further delivered to theprocess. The deaeration reached by means of this apparatus is notefficient enough to provide undisturbed operation of the paper machine,though. In other words, despite the possibility of removing gas fromwhite waters by means of a pumping device according to the publication,this has not succeeded to an extent allowing for eliminating the wirepit, i.e. the white water tank, assisting in the gas-separation.

Thus, despite progressive proposals in order to eliminate the wire pit,one still has to accept the presence of the wire pit in the papermachine approach system. Nevertheless, one does not have to accept thegreat height of the white water tank to be a reason to the energyconsumption of pumping in the paper machine approach system. The whitewater tanks, into which the so-called white waters from the papermachine are collected, have traditionally been relatively big containershaving a height of almost ten meters, located at the bottom level of thepaper mill. The surface level of these tanks, although keeping constantin an individual tank mostly due to overflow, has been greatly alteringin relation to the paper machine. One reason for the altering of thesurface level is the location of the white water tank in connection withthe machine. In case of a so-called fourdrinier machine, the white watertank, which in said case is also referred to as wire pit, is locatedbelow the wire section, whereby its surface level has been relativelylow, due to e.g. constructional reasons. Also, the surface level of awhite water tank arranged aside the wire section or the like (aso-called off-machine silo) is not always as high as it might inpractice be. The big size of the white water tank has been justified onthe basis that the presence of a big buffer tank has been considered apositive factor stabilizing the process. This has also caused both someextra energy consumption, as the first feed pump has had to compensatefor the sometimes-low surface level of the white water tank, andadditional delays in the process caused by the big volume of the whitewater tank.

In the way according to the FI application 981798, it is possible toavoid locating said white water tank in the paper machine approachsystem at the bottom level of the mill, i.e. below the machine level.The solutions described in said application allow for arranging thewhite water tank at the machine level, whereby the gas-separation tankfeed pump aside the white water tank is also located at the machinelevel.

However, said publication mainly concentrates on the possibility ofdecreasing the energy consumption of pumping by utilizing a propellerpump located at the machine level. Said publication only mentions thatat the same time it is also possible to decrease the height of the wirepit and accordingly decrease the time needed for changes of grade.

The present invention handles with problems related to the constructionof a low wire pit and different factors, which have to be taken intoaccount when designing a wire pit.

Firstly, as already stated above, the wire pit must operate as a vesselseparating gas from the white waters, whereby the same rules that applyto other vessels used for gas-separation apply to the construction ofthe wire pit as well, that is the open liquid surface must be as largeas possible. A good starting point may be e.g. that the cross-section ofthe wire pit is kept essentially unchanged.

Secondly, the liquid flow into the wire pit must be kept as laminar aspossible in order not to disturb the gas-separation. Further, becausevarious white waters, i.e. for example having various fiber contents,enter the wire pit, the liquids should have to be directed into the wirepit in such a way that the cleanest fraction of the white waters wouldbe directed to the overflow of the wire pit.

Thirdly, both the liquid entering the wire pit and the liquid dischargedtherefrom should be as non-turbulent as possible, so that the turbulencewould not hamper the separation of gases from the white waters and themixing of thick pulp in the white water in the wire pit discharge.

In addition to that, as a wire pit of small volume is practically inevery respect better and more practical than one of great volume, itshould be possible to locate a small-volume wire pit in an older papermachine approach system as well, that is in an apparatus where both themixing pump and the mixing of thick stock and chemicals are arranged atthe bottom level of the mill. To put it differently, the idealconstruction of a wire pit would be a construction that could bemodified as easily as possible in relation to flows going to and fromvarious locations and, in addition to that, arranged in new systems atthe machine level of the mill and in older systems at the bottom levelof the mill.

A preferable solution according to the present invention is a wire pitmade of modules in such a way that its parts may be located in severalvarious positions in relation to each other.

Characterizing features of the wire pit according to the invention aredescribed in the appended claims.

In the following, the wire pit according to the invention is describedin more detail with reference to the appended figures, from which

FIG. 1 illustrates a prior art paper machine head box approach system,

FIG. 2 is a schematic illustration of another prior art solution,

FIG. 3 illustrates a prior art wire pit,

FIGS. 4, 5 and 6 illustrate a wire pit solution according to a preferredembodiment of the invention,

FIG. 7 illustrates a preferred embodiment of the wire pit solution ofFIGS. 4-6,

FIG. 8 is a schematic illustration of a wire pit solution according toanother preferred embodiment of the invention, and

FIGS. 9a and 9 b illustrate wire pit solutions according to a fifth anda sixth preferred embodiment of the invention.

The paper machine approach system illustrated in FIG. 1 comprises awhite water tank or wire pit 10, a mixing pump 12, a centrifugalcleaning plant 14 with several stages, a gas-separaton tank 16 with itsvacuum devices 17, a head box feed pump 18, a head box screen 20, apaper machine head box 22 and white water trays (not shown). Saidcomponents are arranged in connection with the paper machine 24 andarranged to operate as follows. The fiber material used in paper making,which may comprise fresh pulp, secondary pulp and/or broke, and thefillers which are diluted with so-called white water obtained from thepaper machine, mostly from its wire section, are dosed from the machinechest via flow path 26 into the white water tank 10 to produce paperpulp, in which tank the white waters are collected and which in priorart systems is usually located at the bottom level of the mill as shownin the figure, to produce paper pulp. By means of a mixing pump 12 alsolocated at the bottom level of the mill, said paper pulp is pumped fromthe white water tank 10 into a centrifugal cleaning plant 14 mostusually comprising 4-6 stages and being usually located at the machinelevel K of the mill (the location level of the paper machine with itshead box). The paper pulp accepted by the centrifugal cleaning plant 14flows further under pressure created by said mixing pump 12, assisted bya vacuum of the gas-separation tank 16, into the gas-separation tank 16located at level T above the machine level. The gas-separation tank 16typically comprises an overflow, by means of which the surface level ofthe paper pulp in the tank is kept constant. The paper pulp dischargedfrom tank 16 by means of the overflow flows via pipe 28 downwards intothe white water tank 10 located at the bottom level of the mill belowthe machine level K. From the gas-separation tank 16, the essentiallygas-free paper pulp, wherefrom gas has been removed as completely aspossible by means of vacuum devices 17, flows to the head box feed pump18 located at the bottom level of the mill, which feed pump pumps thepaper pulp to the head box screen 20 also at the bottom level of themill, wherefrom the accepted paper pulp flows to the machine level Kinto the paper machine head box 22. The feed pump 18 most commonly usedis a centrifugal pump, although a propeller pump described in FIapplication 981798 is gaining popularity on the market. Especially thewire pit according to the present invention, when located at the machinelevel of the mill allows for the possibility to use a propeller pumpdescribed in said patent application.

FIG. 2 illustrates just the solution described in said FI patentapplication 981798. It is a new type of white water tank 100 locatedessentially at the machine level of the mill (the main part of the whitewater tank is above the machine level surface and the water surface isclearly above the machine level surface), into which fiber fractions arelead via pipe lines 40-44 and where the surface level is at S₁₀₀. Thefigure shows in phantom lines a prior art white water tank 10 located atthe bottom level of the mill, most usually below the wire section of thepaper machine, the surface level of which tank is at S₁₀, and a feedpump 12. In some cases the level difference between the surfaces S₁₀₀and S₁₀ is several meters, especially in cases when the wire pit islocated under the wire section of the paper machine, whereby the leveldifference may be directly calculated as additional consumption ofpumping energy in a prior art system. Furthermore, a big-sized whitewater tank 10 causes an additional delay in the process. In the solutionaccording to the figure, the level difference dh between the white watertank 100 and the gas-separation tank 16 is less than 9 meters,preferably less than 6 meters, suitably about 4 meters, whereby therequired net positive suction head of the pump 120 is so small that itis quite possible to use a propeller pump.

FIG. 3 illustrates a further prior art wire pit solution. It comprises acylindrical vessel 10 located vertically at the bottom level of themill, at the upper part of which there is/are arranged one or more whitewater channel/s 30 via which the white waters flow into the wire pitessentially to the surface layer of the white water already existingtherein. The surface level of liquid in the wire pit is kept constant bymeans of an overflow 32. The constant surface level ensures that anessentially constant hydrostatic pressure is always maintained at thebottom part of the wire pit. The upper part of the wire pit 10 isfurther provided with a cover 34 and a gas-removal conduit 36 therein,through which the gases separated from the white waters are directed outof the wire pit 10. Both a pipe 26 for thick pulp and pipes 28 forcirculated liquids lead to the bottom part of the wire pit 10. Theseliquids, which are returned to the circulation, are obtained e.g. fromthe gas-separation tank overflow and the centrifugal cleaners as shownin FIG. 1.

FIGS. 4-6 show a wire pit 50 according to a preferred embodiment of theinvention. The wire pit 50 comprises three main parts: an upper part 52,a middle part 54 and a lower part 56. The upper part 52 of the wire pit50 comprises a chute portion 58 which is connected to one or preferablymore white water channels (not shown) coming from the paper machine, andan overflow portion 60. A characteristic feature of the chute portion 58in the embodiment of the figure is that it extends to the whole width ofthe wire pit, as shown in FIG. 4, and that it forms a part of the opengas-separation surface of the wire pit 50. The chute portion isrelatively wide in such a way that its bottom descends towards theoverflow portion. On the one hand, this is arranged because of thereason that by keeping the open surface of the wire pit preferably aswide as the open surface of prior art wire pits it is possible to ensurea sufficient gas-separation capability for the wire pit. On the otherhand, by changing the wire pit construction, compared to prior artapparatuses, so that the bottom of the chute portion 58 is relativelynear to the liquid surface, it is possible to reduce the volume of thewire pit 50 to minimum. Opposite the chute portion 58 at the upper part52 of the wire pit 50 there is an overflow portion 60, which in theembodiment of the figure is about semi-circular. It has to be noted,though, that the proportion of the chute portion to the overflow portionmay vary from the above mentioned even to a great extent. The overflowportion 60 may be considered to be formed of a wall 62 of the upper part52 of the wire pit 50, the upper edge 62′ of which wall determines thesurface level of the liquid in the wire pit 50, and an overflow channel64 located outside of it. One side of the overflow channel 64 is formedof said wall 62 of the wire pit, a bottom surface 66 and an outer sidesurface 68. The outer side surface 68 is preferably located higher thanthe wall 62 of the wire pit 50. In the embodiment of the figure, thebottom surface of the overflow channel 64 descends spirally towards anoutlet conduit 70 for overflow liquid located at its other end.Naturally, the outlet conduit may be located at any location on thebottom of the overflow channel, whereby it is clear that the dischargefrom the bottom of the channel must always be arranged towards theconduit. A further characteristic feature of the upper part of the wirepit is that the height of the outer wall restricting the chute portion58 at its sides is, according to a preferred embodiment of theinvention, essentially the same as the height of the outer side surfaceof the overflow channel 64.

If desired, the wire pit may be provided with a cover and a conduitarranged is therein for leading gases out of the wire pit eitherdirectly to the atmosphere or to a special gas treatment

A further characteristic feature of a preferred embodiment of theinvention shown in FIGS. 4-6 is that the upper part 52 of the wire pit50 formed of both the chute portion 58 and the overflow portion 60preferably ends at a lower edge 72 provided with a flange, which loweredge is in a horizontal position and is preferably round, circular, orat least an equilateral polygon. Naturally, the upper edge 74,preferably also provided with a flange, of the middle part 54 of thewire pit 50 has just the corresponding shape. The purpose of saidcircularity or the corresponding form of equilateral polygon of theedges 72 and 74 is to ensure that the middle part 54 may be positionedin as many positions in relation to the upper part 52 as possible. Itis, of course, possible to think of some rotatable joining methods, butthey might not be justified for economical reasons. Just accordingly,the preferably flanged lower edge 76 of the middle part 54 has the shapeof a circle or an equilateral polygon, as also the preferably flangededge 78 of the lower part 56 on the side of the middle part 54. In thatcase, these parts may also be attached to each other in severaldifferent positions. According to an especially preferred embodiment ofthe invention, the conjunction surface of the middle part 54 and thelower part 56 is positioned in an angle of 45 degrees. The solutions ofFIGS. 6 and 8 show the reason for the 45-degree angle. The cross-sectionof the wire pit 50 converges in the flow direction of the liquid asshown in the figures as evenly as possible towards the outlet opening ofthe lower part 56, which outlet opening is connected either directly orvia an intermediate pipe to a mixing pump feeding the fiber suspensionto the gas-separation device.

If the starting point in mounting the wire pit is that the white waterchutes coming from the paper machine determine the position of the upperpart 52 of the wire pit 50, the rotatability of the middle part 54 inrelation to the upper part 52 to many angle positions makes it possibleto direct the discharge of the wire pit 50 to different directions.Accordingly, the rotatability of the lower part 56 of the wire pit inrelation to the middle part 54 to several different angle positionsallows for further directing the discharge of the wire pit 50. Thus,adaptable construction of the wire pit 50 makes it possible to locatethe mixing pump at the most practical location either at the machinelevel, at the bottom level of the mill or at some other suitable level.

As shown in FIGS. 4-6, each of the parts 52-56 of the wire pit 50 ismade to converge towards the flow direction. Each part has beenconstructed most preferably from one or more conical parts, whenpossible. The aim of the construction is to maintain in the wire pit aflow as non-turbulent as possible in order to ensure a gas-separation asefficient as possible.

FIG. 7 illustrates a construction of the wire pit shown in more detailin FIGS. 4-6 according to a preferred embodiment of the invention. Thisfigure concentrates especially on the positioning and direction of thewalls of different parts of the wire pit. Firstly, in the experiments wehave made we have noticed that the liquid flow discharging from thechute portion 58 of the upper part of the wire pit into the wire pitcreates turbulence in the wire pit, which both decreases thegas-separation from the wire pit and disturbs the smooth flow of theliquid in the wire pit, unless the wall 52″ of the wire pit is slopedboth down- and outwards. In the experiments, the value of the angle a ofFIG. 7 has been determined to be about 5-30 degrees, preferably 10-20degrees. Accordingly, the slope angle b of the wall 52′ positioned as anextension of the chute portion 58 has been determined in the experimentsto be 20-45 degrees, preferably 25-35 degrees, although this angle is tosome extent less significant in view of the total flow than the value ofthe angle a discussed above. Nevertheless, if the angle b is too big,turbulence circulating upwards is formed in the vicinity of the wall,which naturally decreases the flow characteristics of the wire pit. Athird angle to be taken into account is the slope angle g of the middlepart wall 54, which preferably is in the order of 45 degrees, althoughit may vary, depending on the dimensioning of the wire pit, between35-55 degrees. The figure shows as one more preferable method ofdimensioning the wire pit the height dh of the center line of thedischarge opening of the lower part of the wire pit from the wire pitsurface. During the experiments we have noticed that the best result inview of both the volume of the wire pit and its gas-separationcapability, which as such have an opposite effect, is reached when theheight dh alters between 2-5× the diameter of the discharge opening,preferably about 3 times the diameter of the discharge opening. Thediameter of the discharge opening, in its turn, usually varies between400-1000 mm, although, naturally, smaller and bigger dimensions may beapplied in special cases. A further dimensioning principle for the wirepit may be considered to be that the flow velocity of the liquid in thewire pit essentially at the surface level thereof is in the order of0.10-0.15 m/s, wherefrom it is smoothly increased by minimizing thevolume of the wire pit to a velocity of about 1.5 m/s.

FIG. 8 illustrates a wire pit solution according to a second preferredembodiment of the invention e.g. for situations where the wire pitaccording to the invention is used to substitute a prior art wire pitlocated at the bottom level of the mill. In the solution of FIG. 8, thelower part 56 of the wire pit 50 has been turned compared to thesolution of FIG. 6 by 180 degrees, whereby the lower part is directedstraight downwards and may be connected to the mixing pump by means of atraditional pipe elbow.

According to a third preferred embodiment of the invention, the wire pitcomprises two parts only. Compared to the solutions of FIGS. 4-7, thedifference is that in this embodiment the upper and middle parts of thewire pit of FIGS. 4-7 have been constructed stationary, whereby only thelower part of the wire pit may be positioned in different anglepositions in relation to the upper part. This mainly allows for usingone and the same wire pit together with a mixing pump located either atthe machine level or at the bottom level.

According to a fourth preferred embodiment of the invention, also, thewire pit comprises two parts only. Compared to the solutions of FIGS.4-7, the difference is that in this embodiment the middle and lowerparts of the wire pit of figures 4-7 have been constructed stationary,whereby only the upper part of the wire pit may be positioned in severaldifferent angle positions in relation to the lower part. This mainlyallows for using one and the same wire pit with a mixing pump located atvarious directions at the machine level.

FIGS. 9a and 9 b illustrate solutions according to a fifth and a sixthpreferred further embodiment of the invention. In the solutions of thefigures, the white waters entering the wire pit have been separated toat least two parts based on the fiber material or solid materialentrained therein. It is of course possible to utilize several whitewater introduction channels, if desired, but most often two channels areenough. In the embodiment of FIG. 9a, white waters are lead to the chuteportion 58 of the wire pit 50 via at least two channels 82 and 84 sothat the white water flowing via channel 82 is obtained from furtherfrom the head box than the white water flowing via channel 84. Thus, thewhite water flowing via channel 84 contains more solids and fibers thanthe white water in channel 82. At the chute portion, the white waters ofboth channels 82 and 84 are joined in one channel 86, in which the whitewaters are maintained as separate flows to such an extent that thecleaner white water from channel 82 is passed to the overflow edge,whereby material with lower solids-content is passed to furthertreatment therethrough. In the embodiment of FIG. 9b, the chute portion58 leading the white waters into the wire pit 50 has been divided by anintermediate wall 80 to two parts 82′ and 84′, into the first part 82′of which the white waters recovered from further from the head box arelead, which white waters have a low fiber and solids content. The otherpart 84′ receives the more fiber and solids-containing part of the whitewaters, i.e. those obtained from closer to the head box. Anotherdifference compared to the wire pit described before is a deflector 86arranged at the upper part of the wire pit essentially the level of theliquid surface, the purpose of which deflector is to guide the whitewater containing more fiber and solids material to the middle part ofthe wire pit. Thus the fraction that contains the lowest amount offibers and solid material is passed along the edge of the wire pit,which means that the part of the white water that contains less fibersand solid material flows to the overflow. Both solutions lead toessentially decreased fiber losses compared to previous methods, as thecleanest fraction flows to white water filtration and fiber recovery.

A further construction solution worth mentioning is a new type of wirepit embodiment arranged in connection with an older paper machine. Thatis to say, the starting point is a situation where the old wire pit andmixing pump are located at the bottom level of the mill i.e. below themachine level. When changing the location of pulp pipes leading to themixing pump and the lower part of the wire pit is not desired, the newwire pit must be located at the bottom level of the mill, too. But, inorder to fully utilize the possibilities offered by the wire pitaccording to the invention, the wire pit to be used is either that ofFIG. 7 or, as an alternative, that of FIGS. 4-6 located at the bottomlevel. This latter alternative may be carried out so that the wire pitis located at the bottom level and the white waters directed therein areintroduced preferably via several drop legs to the chute portion of thewire pit. In other words, it is preferable to arrange several drop legswhich introduce white waters having various solids or fiber contents tothe chute part of the wire pit, wherefrom they may be lead further intothe wire pit itself e.g. according to FIGS. 9a and 9 b.

A further possible construction stabilizing the operation of the wirepit according to the invention is the use of one or more flow deflectorspositioned in flow direction and arranged inside some part of the wirepit, i.e. either the upper part, the middle part or the lower part,which deflectors do not disturb the flow, but only prevent turbulencewhich might occur therein. Naturally it is clear that said deflectorsmay also form a lattice construction which prevents turbulence onseveral levels.

Finally, it is worth noticing that despite the fact that in prior artsystems the liquid removed from the wire pit via the overflow has alwaysbeen returned to circulation via the white water filter, whereby thewhite water filter has separated usable fiber material from the liquiddischarged at the overflow, the very same arrangement has called for thenecessity to use a relatively large white water filter, because in somecases large amounts of fiber-containing liquid are passed to theoverflow. But this invention presents the use of a fiber-recovery devicearranged in connection with the wire pit overflow, which device may bee.g. a curved screen. In such a case the recovered fiber fraction isquickly returned to the short circulation, e.g. into the wire pit, andthe cleaner liquid is lead e.g. to the white water process. Said fiberrecovery device may even be arranged as a constructional part of thewire pit, whereby the space demand of fiber recovery remains as small aspossible. Advantages of the prescribed solution are, e.g., quickrecirculation of fiber fraction back to the process and decreasedloading of the white water fiber recovery due to the fact that most partof the fiber fraction has already been removed from the white waterdischarged at the overflow.

Further, it is to be noticed that it is preferable to arrange in thewire pit an inlet conduit for makeup liquid, through which it ispossible to introduce make-up liquid into the wire pit in cases when thesurface level in the wire pit tends to lower, i.e. when less whitewaters are obtained than pumped further from the bottom part of the wirepit. The mentioned make-up liquid introduction requires a devicemonitoring the surface level of the wire pit, which device opens themake-up liquid inlet valve when the surface level in the wire pit tendsto lower.

Except for removal of surplus liquid by means of overflow, the wire pitmay also be provided with a discharge conduit for excess liquid and avalve arranged in connection therewith, which valve opens when receivingfrom a level indicator a signal on the rising of the surface level. Thatis, the overflow may be substituted by a discharge conduit, whereby itis possible to arrange in connection therewith separation of fibers fromthe overflow liquid which has been described earlier in thisapplication.

As noticed from the above, a new type of wire pit and apparatus for thepaper machine approach system have been developed, which eliminates manydisadvantages and weaknesses of prior art and solves problems which havebeen hampering the use of prior art approach systems.

What is claimed is:
 1. Wire pit for an approach system of a webformation machine comprising: a chute located in an upper portion of thewire pit for receiving white water, wherein said chute forms a lowersurface of the upper portion of the wire pit, an overflow portionadjacent said chute for stabilizing a surface level of the white waterin the wire pit, a gas separator for separating gas from the whitewater, and a lower portion downstream of the upper portion and saidlower portion having an outlet connectable to a mixing pump, wherein atleast one wall of the wire pit converges downwardly to provide flowdirection of the liquid through the wire pit which deviates fromvertical.
 2. A wire pit according to claim 1 wherein the upper portionof the wire pit has walls sloped downwards and outwards.
 3. A wire pitaccording to claim 1, wherein said chute is divided into to at least twoflow paths for white water streams having different fiber contents.
 4. Awire pit according to claim 1, wherein said chute includes a pluralityof flow paths for white water streams having various fiber contents. 5.A wire pit according to claim 1, wherein the overflow portion includesan upper edge of a wall of the wire pit.
 6. A wire pit according toclaim 3, wherein said chute is connected in a downstream direction to adeflector which directs a flow of white waters having a higher fibercontent to a zone of the wire pit distant from the overflow portion. 7.A wire pit according to claim 1, wherein a wall of the wire pit islocated adjacent the chute, and said wall is sloped downwards andoutwards at an angle in a range of 5 degrees to 30 degrees fromhorizontal.
 8. A wire pit according to claim 1, wherein a wall of thewire pit extends from chute in a flow direction, and said wall descendsat an angle in a range of 20 degrees to 45 degrees from horizontal.
 9. Awire pit according to claim 3, wherein the overflow portion or a flowchannel downstream of the overflow portion includes a fiber fractionseparator for separating fiber from overflow liquid.
 10. A wire pitaccording to claim 9, wherein said fiber fraction separator is a curvedscreen or pressure screen.
 11. A wire pit according to claim 1, whereinsaid gas separator is at the upper portion of the wire pit.
 12. A wirepit according to claim 1, further comprising walls having an insidesurface including at least one deflector positioned in a flow paththrough the pit.
 13. A wire pit for an approach system of a webformation machine comprising: a chute located in an upper portion of thewire pit for receiving white water, an overflow portion adjacent saidchute for stabilizing a surface level of the white water in the wirepit, a gas separator for separating gas from the white waters, and alower portion downstream of the upper portion and said lower portionhaving an outlet connectable to a mixing pump, wherein at least one wallof the wire pit converges downwardly to provide flow direction of theliquid through the wire pit which deviates from vertical, and whereinthe wire pit has a flow path cross-section converging in a flowdirection, and the lower portion is adjustable to a plurality of angularpositions in relation to said upper portion.
 14. A wire pit according toclaim 13, wherein the wire pit further comprises a middle portionlocated between said upper portion and lower portion, and the middleportion is adjustable to a plurality of angular positions in relation toat least one of said upper portion and said lower portion.
 15. A wirepit according to claim 13, wherein the lower portion is connectable tothe mixing pump.
 16. A wire pit, for an approach system of a webformation machine comprising: a chute located in an upper portion of thewire pit for receiving white water, an overflow portion adjacent saidchute for stabilizing a surface level of the white water in the wirepit, a gas separator for separating gas from the white water, and alower portion downstream of the upper portion and said lower portionhaving an outlet connectable to a mixing pump, wherein at least one wallof the wire pit converges downwardly to provide flow direction of theliquid through the wire pit which deviates from vertical wherein saidchute forms a bottom of the upper portion of the wire pit wherein a wallof the wire pit extends from chute in a flow direction, and said walldescends at an angle in a range of 20 degrees to 45 degrees fromhorizontal, and wherein downstream of said wall the wire pit furthercomprises a middle portion between said upper portion and lower portion,and a wall of said middle portion descends at an angle of 35 degrees to55 degrees from horizontal.
 17. A wire pit for an approach system of aweb formation machine comprising: a chute located in an upper portion ofthe wire pit for receiving white water, an overflow portion adjacentsaid chute for stabilizing a surface level of the white water in thewire pit, a gas separator for separating gas from the white water, and alower portion downstream of the upper portion and said lower portionhaving an outlet connectable to a mixing pump, wherein at least one wallof the wire pit converges downwardly to provide flow direction of theliquid through the wire pit which deviates from vertical, wherein saidchute is divided into to at least two flow paths for white water streamshaving different fiber contents, and wherein at least 50% of theoverflow portion is in a zone containing a pulp fraction flow with alower fiber content.
 18. A wire pit for an approach system of a webformation machine comprising: a chute located in an upper portion of thewire pit for receiving white water, an overflow portion adjacent saidchute for stabilizing a surface level of the white water in the wirepit, a gas separator for separating gas from the white waters and saidgas separator is at the upper portion of the wire pit, and a lowerportion downstream of the upper portion and said lower portion having anoutlet connectable to a mixing pump, wherein at least one wall of thewire pit converges downwardly to provide flow direction of the liquidthrough the wire pit which deviates from vertical, wherein the overflowportion has an overflow edge, and a height of the overflow edge,measured from a center line of the outlet opening of the lower portionof the wire pit, is in a range of two to five times a diameter of theoutlet opening.